Fwd: Re: DEC bus transceivers

Al Kossow aek at bitsavers.org
Mon Oct 24 13:08:48 CDT 2016

having half of this conversation not making it from cctech to cctalk is really starting to piss me off

-------- Forwarded Message --------
Subject: Re: DEC bus transceivers
Date: Mon, 24 Oct 2016 13:10:21 -0400
From: allison <ajp166 at verizon.net>
Reply-To: General Discussion: On-Topic Posts <cctech at classiccmp.org>
To: General Discussion: On-Topic Posts <cctech at classiccmp.org>

On 10/23/16 2:50 PM, shadoooo wrote:
> Hello,
> surely the old transceivers are the most compatible solution, however you
> still need to convert the voltages back and forth...
> Plus the solution is not the cheaper, and a little uncomfortable too, as
> you need to find these old chips, hoping not to buy fake chinese duplicates
> (it happened to me more time unfortunately).
> So I was searching a solution with modern components, but not using
> components too much specific and difficult to be found.
> As we need 3.3v logic, but able to work in 5v bus, I'm thinking about 5v
> tolerant standard logic as TI LVC or LVT.
> The problem is that there aren't open drain bus transceivers, but the
> problem could be solved simply using input-only and output-only components,
> connecting two in parallel but opposite direction on bidirectional pins.
> So identifying one or maybe two codes would be enough for all the
> components needed for the board.
> The idea of using bare transistors seems to me too much simple.
> Not that it couldn't work, but it would be almost impossible to satisfy all
> the specifications of the bus in this way... unless you use a more complex
> circuit with precise current sources and resistors to grant correct voltage
> biases, impedances and slew rates, which in the end is a logic integrated
> circuit.
To the last point....  The output of the DEC drivers are simply bare transistors that
had to meet a minimum spec for saturation current/voltage and minimum speed
for the votlage and power of the device.   Modern transistor arrays would be the
first choice for that.    To make it more complex than that is unrealistic and
misses what those old devices were (hint: dirt simple, and somewhat slow,
best of the norm for the day).  Just look at the internal circuits for the devices
They are not complex and look much like open collector TTL on the bus side.
They did not include current sources, and bias systems.  They are not impedence
controlled nor slew rate controlled save for they were fast for the day and there
were slower!

Line receivers any of the devices like LS241 or LS241 have hysteris on the inputs and are
a good match to acceptable bus voltages.  I believe there are CMOS equivilents for that
as well.  I've used the bipolar LS TTL parts rather than CMOS as they have immunity to
latchup and are somewhat more resistant to ESD.


> Andrea

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